The present invention relates to a magnetic toner used in a recording method utilizing electrophotography, electrostatic recording, magnetic recording, toner jet recording, etc.
Hitherto, a large number of electrophotographic processes have been known. Generally, in these processes, an electrostatic latent image is formed on an electrostatic image-bearing member (hereinafter sometimes represented by a xe2x80x9cphotosensitive memberxe2x80x9d) utilizing ordinarily a photoconductive material, the latent image is then developed with a toner to form a visible toner image, and the toner image, after being transferred as desired onto a transfer-receiving material such as paper, is fixed onto the transfer-receiving material by application of pressure, heat, etc., to provide a product copy or print.
As a method for visualizing the electrostatic latent image, there have been known the cascade developing method, the magnetic brush developing method, the jumping developing method, the pressure developing method, the magnetic brush developing method using a two-component type developer comprising a carrier and a toner, the non-contact mono-component developing method wherein a toner on a toner-carrying member in no contact with a photosensitive member is caused to jump onto the photosensitive member, the contact mono-component developing method wherein a toner is transferred from a toner-carrying member pressed against a photosensitive member onto the photosensitive member under the action of an electric field, and the so-called jumping developing method wherein a magnetic toner carried on a rotating sleeve enclosing a magnetic pole therein is caused to jump from the sleeve onto a photosensitive member under an electric field.
As for the jumping developing method, Japanese Laid-Open Patent Application (JP-A) 54-43027, for example, discloses a developing method wherein an insulating magnetic developer (toner) is applied in a thin layer on a developer-carrying member to be triboelectrically charged thereon, and the charged layer of the magnetic toner is moved under the action of a magnetic field to be opposed in close proximity to but free of contact with an electrostatic latent image to effect a development. According to this method, the magnetic developer is allowed to be sufficiently triboelectrically charged by application in a thin layer on the developer-carrying member, and the developer carried under a magnetic force is used for development in a state free from contact with the electrostatic latent image, so that a high definition image can be obtained with suppression of so-called xe2x80x9cfogxe2x80x9d caused by transfer of the developer onto non-image parts.
Such a mono-component developing method, does not require carrier particles, such as glass beads or iron powder, so that a developing device therefor can be small-sized and light in weight. Further, while the two-component developing scheme requires devices for detecting a toner concentration in the developer and for replenishing a necessary amount of toner based on the detected result in order to keep a constant toner concentration in the developer, the mono-component developing scheme does not require such devices, thus allowing a small-sized and light developing device also from these points.
As for electrophotographic apparatus such as printer apparatus, higher resolutions are being desired, e.g., from a conventional level of 300 and 600 dpi to 1200 and 2400 dpi as a technical trend. For these reasons, the developing scheme is also required to be adapted for higher resolution. Further, also copying machines are required to comply with high functionality copying, and digital-mode copying apparatus are becoming predominant. Along with this trend, the latent image formation by using laser beam is predominant together with a requirement for higher resolution. Accordingly, similarly as in printers, higher resolution and higher definition developing scheme is being required.
For complying with such demands, smaller particle size toners having a specific particle size distribution have been proposed in, e.g., JP-A 1-112253, JP-A 1-191156, JP-A 2-214156, JP-A 2-284158, JP-A 3-181952, and JP-A 4-162048. However, smaller-size toner particles are liable to have a larger fluctuation in chargeability, and the control thereof becomes important for accomplishment of the above-mentioned desires. The maintenance of a chargeability also becomes difficult, and the control thereof becomes more important.
On the other hand, the toner image formed on the photosensitive member in the developing step is transferred onto a recording material in a transfer step, and a portion of toner image (transfer residual toner) remaining on the photosensitive member without being transferred is recovered in a cleaning step and stored in a waste toner vessel in a cleaning step. In the cleaning step, a cleaning blade, a cleaning fur brush or a cleaning roller has been conventionally used. From the apparatus viewpoint, however, the presence of such a cleaning device has posed an obstacle to provision of a compact apparatus. Further, from the viewpoints of ecology and effective toner utilization, a system with little waste toner is desirable, and a toner showing a high transferability and causing little fog is desired, correspondingly.
It is well known that the above-mentioned transferability or transfer efficiency is associated with a toner shape and is lowered at a lower circularity (or sphericity) of toner which results in a larger contact area with the photosensitive drum (photosensitive member) and a larger unevenness causing a larger image force due to charge concentration at edges leading to a lower releasability of the toner from the drum. Accordingly, in order to improve the transfer efficiency, it is necessary to increase the toner circularity.
A higher toner circularity is achieved by different methods depending on toner production processes. The production processes for commercially available toners are roughly divided into the pulverization process and the polymerization process. In this pulverization process, toner ingredients such as a binder resin and a colorant are melt-kneaded for uniform dispersion and then pulverized by a pulverizer, followed by classification by a classifier, to obtain toner particles having a desired particle size. The toner particles formed through the pulverization process are accompanied with surface unevennesses since the surfaces thereof are composed of breakage sections formed by the pulverization. Accordingly, a sufficient circularity is not given by only the pulverization, and a surface modification as by mechanical impact or heat treatment for sphering is required as a post-treatment. The polymerization process includes an association and agglomeration process wherein resin particles formed by emulsion polymerization and constituting the binder resin are associated and agglomerated with a colorant and a release agent into a desired particle size to form association-agglomeration toner particles, and a suspension polymerization process wherein a colorant, a release agent, a polymerization initiator, etc., are dissolved or dispersed in a polymerizable monomer to form a polymerizable monomer composition, and the composition is sheared into droplets of a desired size in an aqueous medium, followed by polymerization to provide a suspension polymerization toner. The association-agglomeration toner particles are also accompanied with surface unevennesses attributable to the production process, and require a surface modification post-treatment as by heating of the agglomerated toner particles or seed polymerization by adding a fresh polymerizable monomer composition. The suspension polymerization toner particles are caused to have a shape closer to true spheres compared with toner particle formed through other processes because they have been formed by polymerization of liquid droplets, and therefore provide a toner having a high circularity without a post-treatment. Accordingly, the suspension polymerization process is suitable for providing a high circularity (i.e., toner particles having a high circularity or sphericity). However, in the case of producing a magnetic toner by suspension polymerization, the resultant magnetic toner particles are liable to have a remarkably lower flowability and chargeability. This is because magnetic particles are generally hydrophillic and tend to be present at the toner particle surface. For solving the problem, it is important to modify the surface property of magnetic particles.
A number of proposals have been made regarding surface modification of magnetic material for improved dispersion within polymerization toner particles. For example, treatment of magnetic materials with various silane coupling agents has been proposed by JP-A 59-200254, JP-A 59-200256, JP-A 59-200257 and JP-A 59-224102; and treatment of silicon-containing magnetic particles with silane coupling agents has been proposed in JP-A 10-239897.
By such treatments, the dispersibility of magnetic particles is improved to some extent, but it is difficult to uniformly effect the surface modification (hydrophobization) of magnetic particles, so the coalescence of magnetic particles or the occurrence of unhydrophobized magnetic particles is liable to be caused, thus making it difficult to improve the dispersibility of magnetic particles within toner particles to a satisfactory level. Further, the resultant toner particles are liable to contain different amounts of magnetic particles, so that the toner is liable to show a coloring power and an image quality which are liable to vary depending on environmental conditions and continuation of a continuous image forming operation.
On the other hand, JP-A 7-209904 has proposed a toner comprising toner particles, at which surface the exposure of magnetic particles is completely suppressed.
To summarize the toner organization disclosed in JP-A 7-209904, each toner particle has a structure including a surface layer of at least a certain thickness in which no magnetic particles are present. This means that the toner particle includes a substantial surface layer portion containing no magnetic particles. In another expression, this however means that such a toner particle, when in a small average particle size of 10 xcexcm, for example, includes only a small core volume in which magnetic particles are present, so that it is difficult to incorporate a sufficient amount of magnetic particles. Moreover, in such toner particles, magnetic particles are confined at the core parts and are liable to agglomerate with each other, thus failing to exhibit a sufficient coloring power in fixed toner image.
Further, toners obtained by using monomers having a sulfonyl acid group or similar functional groups have been disclosed in JP-A 63-184762, JP-A 3-56974, JP-A 8-179564, JP-A 11-184165, JP-A 11-288129, JP-A 11-327208 and JP-A 2000-586158. These references however fail to disclose specific examples of magnetic toners at all. JP-A 59-126545 discloses a method of improving the dispersibility of magnetic particles by reaction with a sulfonic acid monomer or a sulfonic acid salt monomer. The resultant toner particles are however accompanied with many magnetic particles present at the surface. As a result of insufficient control of surface magnetic material, the toner particles are liable to have a broad particle size distribution and an insufficient chargeability, so that the toner performances are not satisfactory with respect to image density, image fog and transferability.
JP-A 2000-258953 discloses a method of coating colored particles formed by dispersing a solution of toner ingredients inclusive of a toner binder, a wax and a colorant in an aqueous medium with a resin having an negatively chargeable group, but no specific reference is made to magnetic toners.
A generic object of the present invention is to provide a magnetic toner having solved the problems of the prior art.
A more specific object of the present invention is to provide a magnetic toner capable of exhibiting stable chargeability regardless of environmental conditions, thereby providing high-quality images.
Another object of the present invention is to provide a magnetic toner capable of exhibiting high developing performance and high transferability regardless of environmental conditions, thus providing high-quality images for a long period.
According to the present invention, there is provided a magnetic toner, comprising: magnetic toner particles each comprising at least, a binder resin, an iron oxide and a a sulfur-containing polymer, and inorganic fine powder blended with the magnetic toner particles; wherein
the magnetic toner has a weight-average particle size (D4) of 3-10 xcexcm,
the magnetic toner has an average circularity of at least 0.970, and
the magnetic toner has a magnetization of 10-50 Am2/kg (emu/g) at a magnetic field of 79.6 kA/m (1000 oersted).
According to another aspect of the present invention, there is provided a magnetic toner, comprising: magnetic toner particles each comprising at least a binder resin, an iron oxide and a sulfur-containing polymer, and inorganic fine powder blended with the magnetic toner particles; wherein
the magnetic toner has a weight-average particle size (D4) of 3-10 xcexcm,
the magnetic toner particles retain carbon in an amount of A and iron in an amount of B at surfaces thereof as measured by X-ray photoelectron spectroscopy, satisfying: B/A less than 0.001, and
the magnetic toner contains at least 50% by number of magnetic toner particles satisfying a relationship of D/Cxe2x89xa60.02, wherein C represents a projection area-equivalent circle diameter of each magnetic toner particle, and D represents a minimum distance between a surface of the magnetic toner particle and iron oxide particles contained in the magnetic toner particle.